More than 30 pro-Russian rebels were killed and dozens wounded in Monday`s military assault on the eastern Ukrainian town of Slavyansk, which also claimed the lives of four servicemen, Interior Minister Arsen Avakov said.
"Four of our fighters were killed and 20 were wounded. According to our estimates over 30 terrorists were killed and dozens were wounded," he said, employing the term Kiev uses for the pro-Russian insurgents in control of the flashpoint town.
He said that the rebel fighters included several from Russia and Chechnya as well as Crimea, annexed by Moscow in March.
The Ukrainian military on Monday stepped up its assault on the rebel-held town as part of an operation to seize back control of parts of the east gripped by a pro-Russian insurgency.
On Monday, authorities confirmed the military had suffered casualties of its own and a helicopter gunship was shot down.
An AFP reporter inside Slavyansk said the centre of the city of more than 110,000 people was very calm on Tuesday morning but basic foodstuffs and other items were running increasingly short as the military tightens its noose.
Kiev and the West accuse Russia of fomenting the chaos in eastern Ukraine in a bid to destabilise the former Soviet Republic in the run-up to planned presidential elections on May 25.
Russia denies the charges and accuses the pro-Western government in Kiev -- which it refuses to recognise -- of waging "war on its own people".
Moscow on Monday warned of an impending humanitarian crisis in eastern Ukraine as the military steps up what Kiev calls an "anti-terrorism operation".
Separately on Tuesday, all flights in and out of Donetsk -- a regional industrial hub of more than one million people -- were suspended, according to airport authorities, who gave no reason for the action.